1. Field of the Invention
The present disclosure relates to evaporative purification of liquids and more particularly to harvesting dissolved minerals and salts from ocean water.
2. Description of the Related Art
Evaporation is the process that spontaneously transforms molecules from a liquid phase into a vapor phase. Various factors influence the speed at which molecules will evaporate. Since evaporation only occurs at the surface of a liquid, influencing the factors that allow a molecule to easily break away from the inter-molecular forces bonding the liquid together will speed up evaporation. For instance, if the liquid is heated, then its molecules have a higher average kinetic energy and evaporation will occur much faster.
Evaporation is useful in many industrial applications for separating solids from liquids and/or purifying liquids. For example, lithium, which is used in electric vehicle batteries, is extracted from large salt brine deposits after the liquid has evaporated. In another example, the process of evaporation desalination extracts pure water from sea water for drinking and cooking where fresh water is scarce. In one example of this process, sea water is piped through cooling coils in the top of a vessel before being introduced into the bottom of the vessel. The sea water in the vessel is then heated. The evaporating water molecules are then condensed by the cooling coils and purified water is collected for drinking. Any heating source, such as solar energy, natural gas, oil, nuclear power, or electrical heating (including use of resistive heating elements, microwave systems or other radiative techniques) may be used to heat the vessel and keep the process working continuously.
During the evaporative desalination process, the concentration of salt in the remaining brine solution will increase due to the loss of the water. Eventually, the dissolved salts will crystallize and deposit as a salt scale on the desalination equipment itself. The scale will eventually harden in and on the equipment, making removal extremely difficult, especially inside of pumps, pipes, valves, and other, difficult to clean, areas. We have observed that the salt deposition not only coats the container containing the salt brine, but it also climbs up the walls of such containers and ends up coating areas far away from the actual brine container.
In the book “Evaporation for desalination—scale prevention and removal”, Polytechnic Institute of Brooklyn, New York, N.Y. USA 1966, author Donald F. Othmer teaches that an inexpensive concentrate of a vegetable extract has been found to prevent scale formation in evaporators or boilers using sea water or hard waters from inland sources when used in amounts of one part to 50,000 to 100,000 of water with scale forming constituents which then form flocculent precipitates. Using slightly higher concentrations, scale, even when several inches thick is softened, to give the same sludge which is then blown down. A passivating action of the material appears to prevent corrosion of mild steel by sea water at these same temperatures, up to 350° F., at which scale formation is prevented.
U.S. Pat. No. 4,342,652 teaches that an effective amount of a copolymer of maleic acid or anhydride and allyl sulfonic acid when added to the water to be treated provides increased inhibition of scale formation in evaporative desalination units.
Despite these teachings, further enhancements to the prevention of contaminant scale formation in evaporative purification equipment are needed.
In particular, harvesting dissolved metals and salts from ocean water is an expensive and messy business primarily because of the invasive and corrosive nature of salt. When saltwater evaporates salt crystals form on and bond to adjacent and surrounding materials. If these materials are made of metals, the salt immediately starts to corrode the metals. This salt residue also makes it difficult to effectively and efficiently harvest dissolved metals from ocean water. While some industries are willing to pay the price, in time, energy, and cost, associated with these bodned and corrosive salt crystals, it would be substantially more profitable and more environmentally friendly if it were possible to evaporate saltwater without forming salt crystals that bind to surrounding surfaces.